This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. One of the main virtues of high-field/high-frequency-ESR (HFHF-ESR) over ESR at conventional microwave frequencies is the excellent orientational resolution that it provides for studies utilizing nitroxide spin labels. As a result, at 250GHz, once motion is discernible in the spectrum, one can study in great detail about which axis (or axes) the motion occurs. We have shown that HFHF-ESR slow-motional spectra are much more sensitive to the details of the molecular dynamics than are those at microwave frequencies. A related important feature of the HFHF-ESR studies is the ability to measure very accurately from (near) rigid limit spectra, the magnetic tensors needed for studies of molecular dynamics. Given the superior orientational resolution of HFHF-ESR, we have developed the slowly relaxing local structure (SRLS) model to explore in a consistent manner the interaction of the solute spin label with the dynamic solvent cage (DSC) and its effects on the molecular dynamics. We have chosen OTP as a convenient model system for exploring the enhanced sensitivity of HFHF-ESR spectra to report on details of the local DSC. Although the SRLS model does describe, in a relatively simple manner, the essential features of the DSC, which had been absent from earlier fitting models, it does require increased computational effort. We are using the experience gained from studying the ESR spectra of a variety of spin labels dissolved in the fragile glass former OTP in projects that will study the more complex dynamics of a variety of bio-molecules at the higher frequencies available to us. In this regard, the CSL spin label is an extremely useful probe, given its ready solubility in the cytosolic leaflet. With the assistance of Dr. Hofbauer, we have recently obtained 2D-COSY results on CSL in model systems that give us confidence in the soundness of this approach.